In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Current laser-based lithographic systems frequently rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation (typically in the near-infrared (IR) range) may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer in order to facilitate its removal. Because ablation produces airborne debris, ablation-type plates must be designed with imaging byproducts in mind; for example, the plate may be designed so as to trap ablation debris between layers, at least one of which is not removed until after imaging is complete.
Dry plates, which utilize an oleophobic topmost layer of fluoropolymer or, more commonly, silicone (polydiorganosiloxane), exhibit excellent debris-trapping properties because the topmost layer is tough and rubbery; ablation debris generated thereunder remains confined as the silicone or fluoropolymer does not itself ablate. Where imaged, the underlying layer is destroyed or de-anchored from the topmost layer. A common three-layer plate, for example, is made ready for press use by image-wise exposure to imaging (e.g., infrared or “IR”) radiation that causes ablation of all or part of the central layer, leaving the topmost layer de-anchored in the exposed areas. Subsequently, the de-anchored overlying layer and the central layer are removed (at least partially) by a post-imaging cleaning process—e.g., rubbing of the plate with or without a cleaning liquid—to reveal the third layer (typically an oleophilic polymer, such as polyester).
The commercial viability of any printing system depends critically on the speed at which a printing plate can be imaged, and secondarily on the required laser power. These two parameters are intimately related, as higher laser power results in greater beam fluence, delivering a greater quantity of energy with each imaging pulse. Within limits, higher beam fluence levels increase the rate at which ablation takes place, so that imaging can be carried out at faster speeds—that is, each imaging pulse can be of shorter duration, so the plate can be imaged more quickly.
The relationship between laser power and imaging speed is not strictly inverse, however, and increasing laser power soon leads to diminishing returns, as the responsiveness of the plate imaging layer is constrained by physico-chemical characteristics that limit the rate at which ablation can take place. Moreover, high-power lasers are expensive both to procure and to operate, and can cause damage to the plate beyond the intended results of ablation. Accordingly, increases in imaging speed are desirably realized through improvements in plate characteristics. Such improvements are not easily achieved, however, because increasing exposure sensitivity typically degrades the durability of the plate. For example, sensitivity can be improved by thinning the plate layers or increasing the loading level of an IR-absorbing material, but the result is a more delicate plate structure.